The present invention relates to a condensing optical system for condensing light reflected by a surface of an information recording medium onto a photodetector when information is recorded on the surface of the information recording medium or when the information recorded on the surface of the medium is reproduced, and an information recording/reproducing apparatus comprising this condensing optical system.
In order to perform an optical recording or optical write access to an information recording medium, a beam spot modulated by information is applied on a surface of the medium to change optical characteristics of the medium, or reflected light is detected, which is modulated in accordance with the information recorded on the surface of the medium.
In order to achieve high-density information recording, 1-.mu.m pits are aligned at high density along each track (track width: about 1.6 .mu.m) on a surface of a medium such as an optical disk. Therefore, unless a light beam emitted from a light source such as a semiconductor laser is focused on the surface of the medium by a predetermined focusing lens, with having almost no aberration, accurate information recording/reproduction can hardly be achieved. In order to focus a beam spot at a desired position, even if a disturbance such as surface vibration upon rotation of an optical disk occurs, AF (auto focusing) and AT (auto tracking) must be performed and hence an actuator which can accurately drive an objective lens and a servo system for controlling the drive control of the actuator must be arranged.
An optical system used in an apparatus of this type is constituted by an objective lens system for forming a beam spot on the surface of a medium, and a lens system for condensing light onto a photodetector which is used for detecting information recorded on the medium and for detecting a servo signal for AF and AT control.
In the conventional objective lens system and the condensing lens system, aberrations are corrected well only with respect to a single oscillation wavelength inherent to a semiconductor laser serving as a light source.
However, when an ambient temperature changes or an output from the semiconductor laser changes, a wavelength of the laser beam emitted from the semiconductor laser is considerably changed. For this reason, changes in aberration (generation of aberrations) occur in the lens systems, and accurate information recording or reproduction cannot be peformed.
Japanese Patent Laid-Open No. 3110/1986 discloses a technique wherein an achromatic lens is used in the objective lens system. However, the achromatic lens is merely designed such that light beams of two different wavelengths are focused at an identical position. Therefore, again, this technique cannot prevent an adverse influence on a servo detection system caused by changes in aberration in the condensing optical system due to fluctuations in wavelength.
In the conventional information recording/reproducing apparatus, servo control for AF and AT is unstable due to variations in wavelength. As a result, accurate information recording and reproduction have not yet been realized.